[Caml-list] Re: "ocaml_beginners"::[] string to list

[Caml-list] Re: "ocaml_beginners"::[] string to list

[Issac Trotts]

Christian Schaller wrote:

>Hi everyone!
>
>I'm trying to find a built in function for converting a string to a list of
>characters, similar to SML's explode.  Is something like this available?
>
>Thank you!
>
>- Chris
>
# let explode s =
   let n = String.length s in
   let rec aux k = if k = n then [] else s.[k] :: aux (k+1) in
   aux 0;;
      val explode : string -> char list = <fun>
# explode "foo";;
- : char list = ['f'; 'o'; 'o']

Issac


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Re: [Caml-list] Re: "ocaml_beginners"::[] string to list

[Michal Moskal]

On Mon, Feb 17, 2003 at 02:53:07PM -0800, Issac Trotts wrote:
> Christian Schaller wrote:
> 
> >Hi everyone!
> >
> >I'm trying to find a built in function for converting a string to a list of
> >characters, similar to SML's explode.  Is something like this available?
> >
> >Thank you!
> >
> >- Chris
> >
> # let explode s =
>   let n = String.length s in
>   let rec aux k = if k = n then [] else s.[k] :: aux (k+1) in
>   aux 0;;
>      val explode : string -> char list = <fun>
> # explode "foo";;
> - : char list = ['f'; 'o'; 'o']

Not tail recursive :)

>From my patch to pleac (http://pleac.sourceforge.net):

(* convert string to list of chars *)
let explode s =
  let rec f acc = function
    | -1 -> acc
    | k -> f (s.[k] :: acc) (k - 1)
  in f [] (String.length s - 1)

(* convert list of chars to string *)
let implode l =
  let s = String.create (List.length l) in
  let rec f n = function
    | x :: xs -> s.[n] <- x; f (n + 1) xs
    | [] -> s
  in f 0 l

PS: I know, it doesn't matter much if it's tail recursive or not, nobody
is going to use for data of size when it matters :-)

-- 
: Michal Moskal ::::: malekith/at/pld-linux.org :  GCS {C,UL}++++$ a? !tv
: PLD Linux ::::::: Wroclaw University, CS Dept :  {E-,w}-- {b++,e}>+++ h
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Re: [Caml-list] Re: "ocaml_beginners"::[] string to list

[Brian Hurt]

On Mon, 17 Feb 2003, Issac Trotts wrote:

> Christian Schaller wrote:
> 
> >Hi everyone!
> >
> >I'm trying to find a built in function for converting a string to a list of
> >characters, similar to SML's explode.  Is something like this available?
> >
> >Thank you!
> >
> >- Chris
> >
> # let explode s =
>    let n = String.length s in
>    let rec aux k = if k = n then [] else s.[k] :: aux (k+1) in
>    aux 0;;
>       val explode : string -> char list = <fun>
> # explode "foo";;
> - : char list = ['f'; 'o'; 'o']
> 

let explode s = 
    let n = String.length s in
    let rec aux k accum = if k = n then (List.rev accum) 
                          else aux (k+1) (s.[k] :: accum)
    in
    aux 0 []
;;

See previous discussion.

Brian


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Re: [Caml-list] Re: "ocaml_beginners"::[] string to list

[Issac Trotts]

Brian Hurt wrote:

>On Mon, 17 Feb 2003, Issac Trotts wrote:
>
>  
>
>>Christian Schaller wrote:
>>
>>    
>>
>>>Hi everyone!
>>>
>>>I'm trying to find a built in function for converting a string to a list of
>>>characters, similar to SML's explode.  Is something like this available?
>>>
>>>Thank you!
>>>
>>>- Chris
>>>
>>>      
>>>
>># let explode s =
>>   let n = String.length s in
>>   let rec aux k = if k = n then [] else s.[k] :: aux (k+1) in
>>   aux 0;;
>>      val explode : string -> char list = <fun>
>># explode "foo";;
>>- : char list = ['f'; 'o'; 'o']
>>
>>    
>>
>
>let explode s = 
>    let n = String.length s in
>    let rec aux k accum = if k = n then (List.rev accum) 
>                          else aux (k+1) (s.[k] :: accum)
>    in
>    aux 0 []
>;;
>
>See previous discussion.
>
Good point: we want it to be tail-recursive.  
Just to clean up, and make it more efficient...

let explode s = 
    let rec aux k accum = if k = -1 then accum
                          else aux (k-1) (s.[k] :: accum)
    in
    aux (String.length s - 1) []
;;


Issac

>
>Brian
>
>
>
>  
>



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Re: [Caml-list] Re: "ocaml_beginners"::[] string to list

[sebastien FURIC]

Issac Trotts a écrit :
> 
> Brian Hurt wrote:
> 
> >On Mon, 17 Feb 2003, Issac Trotts wrote:
> >
> >
> >
> >>Christian Schaller wrote:
> >>
> >>
> >>
> >>>Hi everyone!
> >>>
> >>>I'm trying to find a built in function for converting a string to a list of
> >>>characters, similar to SML's explode.  Is something like this available?
> >>>
> >>>Thank you!
> >>>
> >>>- Chris
> >>>
> >>>
> >>>
> >># let explode s =
> >>   let n = String.length s in
> >>   let rec aux k = if k = n then [] else s.[k] :: aux (k+1) in
> >>   aux 0;;
> >>      val explode : string -> char list = <fun>
> >># explode "foo";;
> >>- : char list = ['f'; 'o'; 'o']
> >>
> >>
> >>
> >
> >let explode s =
> >    let n = String.length s in
> >    let rec aux k accum = if k = n then (List.rev accum)
> >                          else aux (k+1) (s.[k] :: accum)
> >    in
> >    aux 0 []
> >;;
> >
> >See previous discussion.
> >
> Good point: we want it to be tail-recursive.
> Just to clean up, and make it more efficient...
> 
> let explode s =
>     let rec aux k accum = if k = -1 then accum
>                           else aux (k-1) (s.[k] :: accum)
>     in
>     aux (String.length s - 1) []
> ;;


 Sometimes the imperative version seems more clear:

let explode s =
  let res = ref [] in
  for i = String.length s - 1 downto 0 do
    res := s.[i] :: !res
  done;
  !res

 Sébastien.
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Re: [Caml-list] Re: "ocaml_beginners"::[] string to list

[Issac Trotts]

Christian Schaller wrote:

>Issac,
>
>Thanks a lot for this.  I am somewhat surprised, since this should be a
>built-in function.  Anyway, maybe you can help me with this one, too:  is
>there also a conversion from character to string or appending a character in
>front or at the end of a string?
>
>- Chris
>
Hi Chris,

Here's a way to make a string from a char:

# let c = 'a';;
val c : char = 'a'
# String.make 1 c;;
- : string = "a"

It's possible to prepend and append like this:

# (String.make 1 c) ^ "foo";;
- : string = "afoo"
# "foo" ^ (String.make 1 c);;
- : string = "fooa"

but it is probably more efficient to use the Buffer 
module (buffer.mli in /usr/lib/ocaml or 
/usr/local/lib/ocaml).

Issac







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